Currently, non-aqueous electrolyte secondary batteries including lithium ion secondary batteries, which are utilized in mobile devices such as mobile phones, have been commercialized. Non-aqueous electrolyte secondary batteries generally have a configuration in which a positive electrode in which a positive electrode active material and the like are coated on a current collector and a negative electrode in which a negative electrode active material and the like are coated on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution is held in a separator. Moreover, the charge and discharge reaction of the battery takes place as ions such as lithium ions are occluded in and released from the electrode active material.
In recent years, however, it has been demanded to diminish the amount of carbon dioxide in order to cope with global warming. Hence, non-aqueous electrolyte secondary batteries with less environmental burden are being utilized not only in mobile devices and the like but also in power supply devices of electrically driven vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), and fuel cell vehicles.
Non-aqueous electrolyte secondary batteries intended to be applied to electrically driven vehicles is demanded to have a high output and a high capacity. Furthermore, non-aqueous electrolyte secondary batteries intended to be applied to electrically driven vehicles are demanded to exhibit cycle characteristics capable of maintaining the capacity even when the charge and discharge cycle is repeated for a long period of time.
However, a density of the negative electrode active material layer is increased as the capacity of the battery is increased, penetration of the electrolyte solution into the negative electrode active material layer is hindered as a porosity in the negative electrode active material layer is decreased, it is partially difficult to donate and receive lithium ions, and the reaction in the negative electrode active material layer takes place ununiformly in some cases. A part of the negative electrode active material is put in an overcharged or overdischarged state by such a local reaction, the generation of lithium dendrite and the reductive decomposition of the electrolyte solution are likely to proceed, and the cycle characteristics deteriorate in some cases.
JP 2013-179101 A discloses a technique to use composite graphite particles in which natural graphite is compounded with carbonaceous or graphite materials as a negative electrode active material of a non-aqueous electrolyte secondary battery. It is described that, by adopting such a configuration, it is possible to obtain a battery exhibiting favorable penetrability of the electrolyte solution and excellent cycle characteristics even when the density of the negative electrode active material layer is increased in order to attain a high capacity.
Particularly non-aqueous electrolyte secondary batteries for electrically driven vehicles are demanded to have an extended cruising distance on a single charge and improved battery durability (cycle characteristics in repeated charge and discharge) in addition to a high capacity and a high output. However, according to the investigations by the inventors of the present invention, it has been revealed that the cycle characteristics cannot be sufficiently obtained in some cases when a flat stacked type battery having a high capacity, a high density, and a large area is fabricated by the technique described in JP 2013-179101 A.
Particularly in a flat stacked type battery having a large area, it is found that it is difficult to uniformly pressurize the active material layer in a plane of the active material layer when the battery is pressurized in the stacked direction, pressure distribution is generated in the plane of the active material layer, the current concentrates on a local site at which the overvoltage is great, decomposition of the electrolyte solution at the time of charge and formation of SEI film on the negative electrode surface occur, and the cycle durability is likely to deteriorate.